About 3,000 people each year visit the mobile Family Van in Boston for free checks of their blood pressure, blood sugar and cholesterol level. The nonprofit clinic affiliated with Harvard University provides simple tests that can give an early warning of an impending health problem or help manage an existing condition, such as diabetes. The van -- which visits six low-income neighborhoods around Boston weekly -- is one of about 2,000 such mobile clinics in the United States. Advocates say the approach can help control the rising cost of healthcare by helping people with chronic diseases to stay out of the emergency room, often the first recourse for inner-city residents.